The A4 Portrait size (2480×3508px)
A4 is the international standard paper size at 210 by 297 millimeters, and at 300 DPI that comes to exactly 2480 by 3508 pixels in portrait orientation. It is the default for letters, resumes, reports, and flyers almost everywhere outside North America, so sizing your artwork to 2480x3508 guarantees it fills a physical A4 sheet edge to edge with no white borders or scaling surprises when it reaches the printer.
The magic of the A series is its 1:1.41 ratio, the square root of two, which means an A4 folded or cut in half gives you two A5 sheets of the same proportions, and two A4s make an A3. Designing at the exact 2480x3508 pixel size respects that geometry so your layout can be scaled up or down the A series without redrawing anything. At 300 dots per inch, body text, photographs, and fine graphics all reproduce crisply rather than looking soft.
For print you should also mind bleed and margins. If your design has color or imagery running to the edge, extend it a few millimeters past the 210x297mm trim so nothing shows white after cutting, and keep important text inside a safe margin from the edge. Home and office printers usually add their own small unprintable border, which is worth checking. Everything here resizes 100% in your browser with no upload and for free, so your files stay private. Confirm bleed, DPI, and margins with your printer before a professional run.